This application claims priority under 35 U.S.C. xc2xa7xc2xa7 119 and/or 365 to 9903629-5 filed in Sweden on Oct. 8, 1999; the entire content of which is hereby incorporated by reference.
The present invention relates to silicon bipolar transistors, especially low-voltage high-frequency transistors for use in mobile telecommunications.
Integrated circuits using bipolar transistors play a major role for modern telecommunication systems. The circuits are mostly used for analog functions, e.g. for switching currents and voltages, and for high-frequency radio functions (mixers, amplifiers, detectors etc.). For mobile telecommunication applications (e.g. handsets), the circuits are operated at low supply voltage ( less than 3.5 V) to conserve energy and enable battery operation.
To achieve good high-frequency properties of the transistors, the base must be made very narrow. Several problems from the physics as well as the practical point of view arise. The doping of the base must be carefully tuned to give reasonable beta (current gain), not too high doping in the emitter-base junctions (low BVebo will follow otherwise), enough doping to withstand voltage applied over the base without going into punch-through breakdown, the Early voltage should be high, base resistance should be low, etc.
The base is usually formed by ion implantation of boron. For a thin base, the preferred shape of the doping is a box, but with ion implantation, a smooth, almost half-triangular shape is usually obtained. A more advanced approach to the profile problem is to epitaxially deposit an in-situ doped base layer, thus obtaining a box profile structure. However, this will only improve some of the above-mentioned parameters.
Epitaxial-base transistors can be further improved by using Sil-xGex (0 less than xc3x97 less than 0.2 typically) as material in the base. The SiGe-base transistor is an heterojunction bipolar transistor (HBT). The difference in band-gap between SiGe and Si is used to further reduce the base width, while maintaining a reasonable beta and base resistance, in order to increase the high-frequency properties of the transistor. The SiGe-base transistor may also have increased beta-Early voltage product (hFE.VA) as well as reduced temperature dependence of hFE.
Indium has a reasonably low diffusion coefficient and implant characteristics suitable for obtaining narrow doping layers. Indium is a p-dopant and could possibly be used as a replacement for boron in the base of the bipolar transistor. The most interesting difference compared to boron is the property of indium as xe2x80x9cnon-shallowxe2x80x9d acceptor. At typical transistor operating temperatures, only a fraction of the indium acceptor states are ionized or active (referred to as xe2x80x9cimpurity freeze-outxe2x80x9d [1]). As a result, the effective Gummel number is reduced and current gain is increased.
Another interesting aspect is that indium-base bipolar transistors should be less susceptible to the base-width modulation (i.e. Early effect) being caused by the reverse-biased collector-base junction (normal state of operation), because indium acceptor states falling within the collector-base depletion region will ionize and prevent the quasi-neutral base from being depleted by the reverse-biased junction. This should also reduce the voltage-dependent capacitance variation of the base-collector junction, which contributes to the non-linearities in the high-frequency transfer characteristics.
All bipolar silicon transistors exhibits a fairly high positive temperature coefficient for beta, i.e. beta increases with temperature. This temperature dependence leads to many difficulties and tradeoffs in circuit design. Furthermore, for bipolar power devices, the positive sign of the coefficient leads to thermal instability problems which is usually solved using ballasting emitter resistors, but at the expense of reduced performance and increased costs. Bipolar transistor that use indium in the base will have a reduced temperature dependence of beta, since at high temperatures more acceptor states will be ionized, thus increasing the effective base doping. This will reduce an increase in beta with temperature.
In U.S. Pat. No. 5,681,763 by Ham and Kizilyalli, a method to make bipolar transistors with an indium-doped base is described. In the patent, boron and indium dopings for the base are discussed in details (mainly for the purpose of improving the Early voltage of the transistor), but the effect of combining the two species is never discussed, although mentioned in the process flow in the text and the first claim, that when doping the base with indium, it may also be desirable to dope the base with boron.
The indicated range of indium dose (1.1012-1.1015 cmxe2x88x922) in the cited patent is too wide. At high doping level, the indium, which is a deep level impurity, will create a base recombination current, which will lead to leaky transistors and low low-current beta. The dose range instead have to be kept below 1.1014 cmxe2x88x922 to suppress this effect. At high levels of indium a transient enhanced diffusion (TED) of both boron and indium will occur completely destroying any advantages.
Bipolar transistors with indium-implanted base were experimentally verified [2, 3] using 0.5 xcexcm and 0.25 xcexcm BiCMOS technologies. The most important improvement was the increased Early voltage, and thus the improved beta-Early voltage product (hFE.VA product). The beta and base resistance however was increased more than ten times. This makes it more or less impossible to use the suggested device in any realistic radio circuit design.
In high-frequency bipolar transistors for use in telecommunication, a factor beta in the range 50-150 is preferred. In [2], an increase in beta from 120 to 1600 was obtained, and in [3] an increase from 120 to 1300 was obtained. To obtain a lower beta, very high indium concentration is needed and will lead to severe recombination of minority carriers in the base.
Low base resistance (RB) is a very important parameter. It affects fmax (œ1/{square root over (RB)}), which is an important high-frequency parameter, and the noise in the device (œ{square root over (RB)}). In [2], the base resistance increased up to 21 times and in [3] 14 times compared to similar boron-base transistors.
Therefore there is still a demand for a method to improve the characteristics of bipolar silicon high-frequency transistors for improving the beta-Early voltage but without obtaining too high beta and/or too high base resistance.
The present disclosure describes a method to improve the characteristics of bipolar silicon high-frequency transistor by adding indium into the base of the transistor. Only replacing the boron in the base with indium improves the beta-Early voltage product, but at the price of high beta and high base resistance.
Instead, it is here suggested to combine boron and indium doping profiles in the base to obtain a transistor with most of the properties of boron-base transistors preserved, but with some parameters improved because of the added indium. This xe2x80x9cdouble-profilexe2x80x9d or xe2x80x9cindium-enhancedxe2x80x9d transistor will exhibit improved beta-Early voltage product and lower temperature dependence of beta, but will otherwise preserve the advantageous properties of boron-base transistors.
For this to work satisfactory, the indium profile must however be contained within the boron profile in such a way that the beta and the effective base width are not dramatically affected, otherwise the low and high frequency properties will be degraded.